Heavy duty trucks normally operate extremely fuel rich when maximum power is required. When emissions controls are mandated for these vehicles in the near future, extremely robust catalysts will be required since the large amounts of carbon monoxide and unburned hydrocarbons in the exhaust will result in extremely high catalyst operating temperatures at least during the maximum power portion of the cycle. Typically, heavy duty gasoline engines are operated at rich air-to-fuel ratios for maximum power output resulting in high levels of carbon monoxide in excess of 3% and unburned hydrocarbons greater than 100 ppm in the exhaust. In contrast, modern automotive exhaust usually contain about 1 to 2% carbon monoxide and 300 ppm of unburned hydrocarbons. Thus, while peak catalyst operating temperatures in the neighborhood of 700.degree. to 800.degree. C. are common in automobiles, inlet temperatures of 700.degree. and peak operating temperatures around 1300.degree. C. are expected for heavy duty trucks. Further, average catalyst operating temperatures for heavy trucks of over 800.degree. C. are anticipated for much of the time that the engine is operating at 75% of maximum load. Thus, it can be appreciated that the catalysts to be used for heavy duty trucks will need to be capable of withstanding temperatures far in excess of the temperatures presently encountered in automotive catalysts.
In conventional monolithic catalysts, the conventional alumina supports for the catalytic metals are stabilized with oxides such as alkaline earths, rare earths, zirconium oxide or silicon dioxide. However, the stabilized aluminas presently used are not normally sufficiently stable to provide the required surface area after prolonged exposure to high temperature.
Platinum metal catalysts, especially palladium, supported on ceria modified alumina have found particular utility as pollution abatement catalysts as described in U.S. Pat. No. 3,993,572. As disclosed in U.S. Pat. No. 3,956,188 and U.S. Pat. No. 4,170,573, similar catalyst compositions have been found useful for high temperature applications including catalytically oxidizing a fuel in a combustion operation for purposes of energy production. However, these catalysts will not necessarily have sufficient durability for heavy duty applications.
Simple bare metal oxides have been demonstrated to be useful in CO oxidation as reported in J. Catalysis, 12, 364 (1968) by Shelef, et al. However, these oxides are often not sufficiently stable thermally and they tend to react with alumina to form either an aluminate spinel (in the case of Mg, Mn, Co, Ni, Cu, Zn and Fe) or a perovskite (in the case of La, Y and Nd), thereby accelerating deterioration of the alumina support, Thus, even though catalysts for oxidation of carbon monoxide are well known, it has not been known how to achieve this result repeatedly after exposure to high temperatures and the art has continued to search for catalyst compositions which are stable at higher temperatures.